1. Field of the Invention
The present invention relates to an apparatus for balancing rotary bodies, such as the wheels of a motor vehicle. More particularly, the invention relates to a parameter entry device for electronically measuring the parameters of a wheel to be balanced.
2. Description of Related Art
Wheel balancers are well known in the automotive service equipment art. For example, U.S. Pat. No. 4,046,017, which issued to Hill and is owned by the assignee hereof, discloses a motorized wheel balancer comprising a rotatable shaft upon which the wheel to be balanced is mounted and a pair of force transducers positioned adjacent the shaft for measuring the forces imparted to the shaft by the unbalance in the rotating wheel. These wheel balancers typically comprise computer processor means for processing the force transducer outputs to determine the magnitudes and angular positions of the unbalance forces appearing on both the inner and outer rims of the wheel. In order for the processor means to determine the magnitudes of the unbalance forces, certain measurements relating to the wheel to be balanced must be provided, such as the distance between the force transducers and the inner rim of the wheel, which is commonly referred to as offset, and the width and diameter of the rims at the point where the correction weights are to be attached.
Some prior art wheel balancers employ electronic parameter entry devices to aid the operator in measuring the offset and diameter of a wheel to be balanced. These devices typically comprise an elongated rod which is mounted parallel to the rotatable shaft and includes a transverse arm connected to the end of the rod closest the wheel. The rod is both extendible and rotatable to bring the distal end of the arm into contact with the rim of the wheel on which a correction weight will be attached, and electronic means, such as potentiometers, are coupled to the shaft to measure the linear extension and rotation of the shaft, from which the processor means can compute the offset and diameter of the wheel.
The potentiometers used in prior art parameter entry devices are analog devices which can be adversely affected by the normal changes in temperature and humidity experienced in a wheel service station environment. In addition, in wheel balancers employing digital microprocessors as the processor means, additional circuitry must be provided to digitize the output of the potentiometer. Furthermore, parameter entry devices may become out of calibration under the normal, typically rigorous use of the wheel balancer.